The present invention relates to a reducing agent supply apparatus for injecting in an exhaust pipe liquid reducing agent used for purifying exhaust gas from an internal combustion engine, a method for controlling the reducing agent supply apparatus, and an exhaust gas purification apparatus.
Conventionally, as one aspect of an exhaust gas purification apparatus for removing nitrogen oxides (NOx) in exhaust gas from an internal combustion engine, a urea selective catalytic reduction (SCR) system is practically used which includes: a reducing agent supply apparatus for injecting urea aqueous solution in an exhaust pipe; and a reducing catalyst that adsorbs ammonia produced by decomposition of urea in urea aqueous solution then selectively reacts NOx in incoming exhaust gas with the ammonia to reduce the NOx.
The reducing agent supply apparatus used in such a urea SCR system includes: a storage tank for storing urea aqueous solution; a pump for sucking up and pumping urea aqueous solution stored in the storage tank; and a reducing agent injection valve for injecting the pumped urea aqueous solution in the exhaust pipe.
Here, urea aqueous solution as a liquid reducing agent is adjusted to a concentration having the lowest solidifying point (e.g., 32.5% concentration with the solidifying point of about −11° C.) so as not to freeze even in cold climates. However, when the concentration increases due to evaporation of solvent in urea aqueous solution or the like reason, the solidifying point increases to make urea aqueous solution more likely to solidify. When urea aqueous solution solidifies in the reducing agent injection valve or a reducing agent passage while the internal combustion engine is stopping, the next injection of urea aqueous solution may not be properly performed. So, while the internal combustion engine is stopping, a purge process of collecting urea aqueous solution in the reducing agent supply apparatus into the storage tank is performed (e.g., see JP-A-2010-7560).